Question

When predicting the phenotypic ratio of a crossing between two heterozygous fruit flies, the ratio was shown to be 9:3:3:1. What would the parental fruit fly's genotype have been if the resulting offspring had a 1:1:1:1 ratio instead?

Answer 1

A phenotypic ratio of 1:1:1:1 indicates a cross of two parents that are heterozygous for different traits, such as AaBb x AABB, where one parent has dominant alleles and the other has recessive alleles for the traits considered.

Step-by-step explanation:

Predicting the phenotypic ratio of offspring from genetic crosses involves understanding Mendelian genetics. When a student asks about the parental genotypes that would give a 1:1:1:1 phenotypic ratio instead of 9:3:3:1, we are dealing with a dihybrid cross. The 9:3:3:1 ratio emerges from crossing two heterozygous parents (AaBb x AaBb). However, a 1:1:1:1 ratio suggests a cross between two different types of parents that are both heterozygous, but for different traits (e.g., AaBb x AABB), where one parent carries both dominant alleles and the other parent carries both recessive alleles for the traits in question.

Using the Punnett square method depicted , we can illustrate this concept further. Typically, in such a cross, each genetic combination has an equal chance of occurrence, which would result in the hypothesized 1:1:1:1 ratio. However, if we have a heterozygous state as in the initially described 9:3:3:1 ratio (from the parents being AaBb x AaBb), the probability of inheriting a combination of dominant and recessive traits from both parents follows Mendel's laws of segregation and independent assortment, leading to the classic dihybrid ratio.